We've seen methane rain gleaming on the icy plains of Titan

We’ve spotted the rainy season near the north pole of Saturn’s moon Titan. Images taken by the Cassini spacecraft before it plunged into the gas giant planet have revealed a huge shining plain of wet ground that likely came from a summer rainstorm on Titan – the only other world where we know it rains.

Titan is in some ways a bizarre version of Earth, complete with seas, lakes and rivers. The crucial difference is that it is not water flowing on Titan, but liquid methane and other hydrocarbons.

When Cassini arrived at the Saturn system in 2004, it was summer at Titan’s south pole, and there were signs of clouds and rainfall there. Models predicted that the stormy season should slowly move towards the north pole over the next decade or so, but by 2014 only a few patchy clouds had materialised there.

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Rajani Dhingra at the University of Idaho and her colleagues found the first evidence of rain near Titan’s north pole in Cassini images from 2016. On 6 May that year, the pole was blanketed in clouds. During the next pass over the area on 7 June, two Titan days later, the clouds were gone and the area was shining bright. Three Titan days after that, on 25 July, the gleam was gone.

When the researchers analysed the light reflecting from the 120,000 square kilometre plain, they found that it was likely from a wet, slightly rough surface – similar to a damp pavement after a rain shower. The wetness was probably due to methane rain that later flowed into Titan’s hydrocarbon lakes or evaporated away.

“It was later than we expected, and we don’t know why,” says Dhingra. It could be a sign that we do not quite understand the seasons on Titan. “Northern summer is not exactly like southern summer.”

Standing in such a rainstorm would be a strange experience, not least because Titan’s average surface temperature is about −179 °C. “Raindrops on Titan should fall extra slowly because of the low gravity and thick haze, so if you stood there in the rain, you would feel each drop falling on you,” Dhingra says.